Aimed towards developing a molecular mechanism for genetic recombination in yeast, the proposed genetical research centers on issues related to the formation and repair of heteroduplex DNA. Different but complementary approaches will a) assess the role of specific base-pair mismatches. Gene conversion and postmeiotic segregation will be analyzed among unselected tetrads of diploids carrying different nonsense mutations in the same codon UAA/UAG or UAG/UGA. b) Characterize the mechanism of action for mutants known to enhance prototroph or single site conversion frequencies. c) Generate systems selective for mutants deficient in repairing recombinational heteroduplex DNA. Such mutants might exhibit normal genetic maps but display increased PMS over the entire genome at the expense of gene conversion. d) Document the effect of known deletions on postmeiotic segregation and other conversion parameters. e) Survey homozygous rad mutants, singly and in combinations, relative to recombination and recombination repair in unselected tetrads. f) Test for the existence of correctional polarity in meiotic recombination. g) Quantify conversional and PMS disparities and test these for correlations with mutagenic origins of the various genetic alterations. h) Develop workable systems for analyzing gene conversion in unselected mitotic cells. Any comprehensive recombination scheme must consider such fundamental data. i) Extend or complete genetic analysis of second site alterations in hisl that generate feedback resistance and histidine excretion or novel types of interallelic complementation. Some studies at the enzyme protein level are projected, especially those concerned with subunit aggregations, in vitro complementation, and allosteric site modification.